Patent Application
20240102193
The present non-provisional patent application claims priority, as per Paris Convention, from Japanese Patent Application No. 2022-152878 filed on 2022 Sep. 26, the disclosure of which is hereby incorporated by reference herein in its entirety.
This invention relates to gold electroplating solutions used in the manufacture of wiring circuits for high-density semiconductor devices or printed circuit boards or in the manufacture of semiconductor circuits, and particularly relates to gold electroplating solutions for forming void-less Via-filling (i.e., compactly packed deposit) with U shaped layer-stack structure (U shapes when seen in cross section) inside drilled holes, wherein only bismuth is contained preferentially over thallium in the gold deposit.
Electrodeposition reactions with gold electroplating solutions based on gold (I) sulfite complexes and sulfates have been studied since the 1950s, and various metal crystal conditioners have been investigated to smooth the electrodeposited gold layer. For example, Japanese Patent Publication No. 10-251887 published in 1998 (hereinafter Patent Document 1) discloses a non-cyanide electro-gold plating bath containing gold sulfite salt (Na3[Au(S2O3)2]) and 1 to 100 g/L acetylcysteine as a complexing agent. This electro-gold bath is said to be capable of being added with known metallic elements utilized in gold cyanide electroplating baths, such as silver, copper, indium, iron, nickel, cobalt, lead, tin, cadmium, antimony, bismuth, zinc, arsenic, thallium, selenium, tellurium and cesium.
In addition, International Publication No. 2014/054429 (hereinafter Patent Document 2) reports on an alkaline non-cyanide electrolytic gold plating solution containing a crystal modifier suitable for gold bumps and gold wiring. This gold plating solution is described to contain a gold source consisting of sodium gold sulfite, a conducting salt composed of sulfite, platinum group sulfate, and a crystal modifier. Metal elements such as thallium, bismuth, lead, and antimony are said preferable for this crystal modifier, and a thallium crystal modifier is named in the examples.
On the other hand, as the density of fine conductor circuits increased in the manufacture of silicon wafer substrates for semiconductor devices and printed circuit boards, it has become necessary to provide blind via holes (sometimes called vias, grooves, or trenches) with a diameter of 1 to 50 μm and a depth of 1 to 100 μm in the middle of conductor circuits, such as printed wiring boards (PWBs) with integrated circuits and through-silicon vias (TSVs), and wafer-level packages (WLPs). Initially, a technique was developed to fill the inside of the drilled holes with conventional gold electroplating solution by pre-treating the flat surface of the substrate, which is not concave, with a non-conductive treatment, as described in U.S. Pat. No. 6,410,418 (hereinafter Document 3).
The amount of gold required to fill the interior of the drilled hole with gold electroplating usually requires a solution containing about 103 times the amount of Au ions as the plating solution present in the interior.
The gold electroplating process can be detailed as follows: the direct current emitted from the anode causes the gold ions of the gold sulfite (I) complex in the gold electroplating solution and molecules such as metal salts and sulfite ions to move toward the surface of the cathode consisting of the object to be plated. In the normal gold electroplating process, Au ions are preferentially electrolyzed and deposited in the form of gold electrolyte at the stronger current density distribution in the current flow. As shown in the right-hand part in
In a normal gold electroplating process, the current flow concentrates at the opening of the drilled hole and thus the current density distribution becomes denser there, as shown at A in
However, deeper in the drilled hole, the sparser the current density distribution becomes. Hence, if the opening of the drilled hole is narrow or the sidewalls of the drilled hole are tall, it becomes difficult for the current to flow at the bottom of the drilled hole, making it impossible to initiate an electrolytic deposition reaction there. In the area B shown in
Since the circulation of gold electroplating solution inside the drilled hole is poor, various attempts have been made to improve the flow of the solution, including mechanically shaking the object to be plated, or adding various additives such as surfactants or sulfur-containing organic substances to change the properties of the solution. However, even if Via-filling plating is performed with existing gold electroplating solutions and/or gold electroplating methods, these do not create a sufficient current flow toward the bottom sidewall at B in
To solve this problem, a new gold electroplating method called super conformal filling method was proposed. For example, an example in U.S. Patent Application Publication No. 2005/0092616 (hereinafter Patent Document 4) describes a superconformal filling method in which a silicon wafer is rotated in a plating bath containing sodium mercaptopropanesulfonate and thallium and plated in pulses at periodic intervals of 2 ms on and 8 ms off (paragraphs 0053-0056,
Also, U.S. Patent Application Publication No. 2019/0093248 (hereinafter Patent Document 5) discloses a method of filling recessed features from their bottom with an electrolyte containing Au(SO3)23− anion, SO32− anion and Bi3+ cation.
However, convective transport of gold electroplating solution inside the drilled hole by super-conformal filling method or stepping of potential does not control the current density distribution on the sidewall of the drilled hole. As shown at B of
When a large number of plated materials such as a large number of silicon wafers or printed circuit boards are put into a gold electroplating solution in large quantities, the electrolytic deposition conditions of individual plated materials vary greatly. In the super-conformal filling method, it is practically impossible to uniformly control the fluctuation range of plating conditions because the equipment becomes too complicated and expensive. In other words, the super-conformal filling method has the disadvantage that the plating solution tends to become unstable during the electroplating process, making it unsuitable for mass-produced products. In addition, the gold plating film of pulse-electroplated gold deposit has inferior plating film properties such as elongation and tensile strength, which also causes the inconvenience of extra costs for heat treatment in the post-process.
[Patent Document 1] Japanese Patent Application No. 10-251887
[Patent Document 2] International Publication No. 2014/054429
[Patent Document 3] U.S. Pat. No. 6,410,418
[Patent Document 4] U.S. Patent Application No. 2005/0092616
[Patent Document 5] U.S. Patent Application No. 2019/0093248
In view of the above circumstances, the present invention was made to provide a cyanide-free gold electroplating solution which produces a deposit wherein bismuth is contained preferentially over thallium in such areas where the amount of current flow has been sufficient. The inventors noticed that the reaction rate of electrolytic deposition from Au ions to gold particles differs depending on the type of metal additives in gold electroplating solution. In view of the above circumstances, the present invention was made to Via-fill, using a cyanide-free gold electroplating solution, the areas of sidewalls and the bottom of the drilled hole where electric current does not flow sufficiently. The present inventors noticed that the deposition reaction rate from Au ions to gold electrodeposits differs depending on the types of metal catalysts in cyanide-free gold electroplating solution. Due to the different reaction rates of two or more metal additives, the catalytic reaction of one metal additive becomes dominant and that of the other metal additives become inferior in the region of sufficient current flow, generating a specific catalytic inhibition reaction.
The present invention was also made to provide a gold electroplating solution with which a compactly packed Via-filling deposit with a U-shaped stacked structure (when seen in a cross-section) is obtained in the region of the drilled hole where the electric current does not flow sufficiently. The inventors found that in a gold electroplating solution with two different metal additives, the adsorption and desorption reactions of the two metal additives cooperate to produce compactly packed Via filling deposit with a U-shaped stacked structure in cross section in regions inside the drilled hole where the current does not flow sufficiently, as a result of the different reaction rates of the two metal additives.
The inventors investigated various metal compounds and found that in gold electroplating solutions based on gold (I) sulfite complexes, sulfite or sulfites, and sulfates, a metal additive combining a thallium compound and a bismuth compound is the most effective combination for mixing bismuth in the gold deposit and obtaining an optimum Via-filling result inside the holes perforated by drilling. The adsorption and desorption reactions of the bismuth and thallium catalysts occurred at the upper corners of the perforation holes, where the current density distribution was the highest. When a given amount of thallium (Tl+) and bismuth (Bi3+) ions coexisted, the gold deposit filling rate inside the perforation holes tended to improve as the concentration of sulfite ions (SO32−) increased.
The inventors have found that when the concentration ratio of thallium ion (Tl+) and bismuth ion (Bi3+) in a given amount is varied, thallium, which is toxic to the human body, may or may not be entrained in the gold electrodeposit on the ppm order. We also found that in the gold electroplating solution where only bismuth is preferentially entrained in the gold deposit over thallium, the Via-filling inside the drilled hole is densely formed in a U-shaped stacked structure in cross section, leading to the completion of this invention. This specification also includes the invention of a gold electroplating method using this gold electroplating solution and a method for manufacturing this gold electroplating solution.
As is known in the electrolytic deposition reaction of the gold electroplating process using a normal DC power supply, metal additives such as thallium and bismuth compounds act as crystal modifiers to flatten the Au metal in the region where sufficient current flows. This is illustrated in A of
As shown in A of
The inventors noticed that the liquid resistance of the gold electroplating solution with more bismuth catalyst content than thallium catalyst content (solid line) is different from that of the gold electroplating solution with more thallium catalyst content than bismuth catalyst content (dashed line), as shown in
When gold electroplating work is performed using such gold electroplating solution, it was found that the inside of the drilled holes can be filled with Via-filling without voids or clefts. The inventors speculated that the different reaction rates of the bismuth and thallium catalysts, which reduce and deposit Au ions, result in the reduction and deposition of Au ions by both catalysts. The current flowing in the Au electroplating solution decreases continuously as it advances to the bottom of the perforation hole. This would create a site where cooperative adsorption/desorption reactions between bismuth catalyst activity and thallium catalyst activity occur on the side wall of the perforation hole, and this would be the starting point for the catalyst suppression reaction to spread inside the perforation hole. Comparison of Vb in
The compactly packed Via-filling shown at (b) of
The bismuth and thallium catalysts adsorbed on the gold (I) sulfite complexes are repeatedly regenerated during the gold electroplating process, so that the catalyst suppression reaction of the present invention continues even if amounts of the catalysts are in trace, and thus excess Au ions are stimulated. In other words, once the catalytic deposition suppression reaction by the bismuth catalyst occurs, the Au deposit spreads to the cathode surface where the current flow is normally weak and the current density distribution is sparser. Inside the drilled hole, U-shape-layers packing (filling) as seen in cross section is compactly formed by virtue of the autonomous thallium catalytic action precipitation reaction, as shown at (b) of
It is an object of the present invention to provide a gold electroplating solution that forms a flat gold plating layer of gold deposition without thallium elements. It is also an object of the present invention to provide a gold electroplating solution that forms such interior of drilled holes by dense Via-filling deposit as to have a U-shaped layered structure in cross section. It is also an object of the present invention to provide a gold electroplating solution that forms a drilled hole filled with a flat gold plating layer and dense Via-filling deposit free of thallium elements by bismuth and thallium catalysts. It is also an object of the present invention to provide a gold electroplating solution that can densely fill the interior of the drilled hole regardless of the surface morphology of the plated object and that can perform electroplating work under the same current density conditions as those of conventional flat circuit boards.
The above object of the present invention also includes providing a gold electroplating method using a gold electroplating solution that can preferentially catalytically deposit gold electrodeposits at locations where the current density distribution of the plated material is sparse and can Via-fill the internal cross section of the drilled hole in a U-shape. The above-mentioned object of the present invention also includes a gold electroplating method using gold electroplating solution in which catalytic deposition reaction by bismuth catalyst and thallium catalyst preferentially starts at the part of the drilled hole where the current density distribution is sparse, electrolytic deposition reaction preferentially starts at the part where the current density distribution is dense, both reactions are switched autonomously and gold deposits are completely Via-filling without defect. The method includes providing a gold electroplating method using a gold electroplating solution in which the gold deposits are completely filled with Via-filling without defects while both reactions are switched autonomously.
The purpose of the above gold electroplating solution also includes providing a flat gold-plated layer of gold deposits containing bismuth elements formed by the gold electroplating solution of the present invention. It also includes providing drilled holes with a U-shaped layered structure in cross section filled with compactly packed Via-filling deposit by the gold electroplating solution of the present invention. Furthermore, the invention also includes the object of providing a method for producing the gold electroplating solution described above. However, these tasks and purposes are exemplary only, and the scope of the invention is not limited thereby.
A cyanide-free gold electroplating solution for forming, inside a drilled hole, a gold deposit having bismuth preferentially entrained over thallium and having a U-shape-stacked structure when seen in vertical cross section, comprising the following basic elements and additional elements, wherein, the basic elements are
The following embodiments also form part of the gold electroplating solution according to the present invention.
Here, in the range of gold (I) sulfite complexes in (1) through (4), (a) above, the lower limit value of 3 g/L or more as gold (Au) element is preferred to form a stable gold plating film. On the other hand, the upper limit value of 30 g/L or less is preferred because gold bullion is expensive and the amount of pumping out due to adhesion of the solution to the material to be plated increases. The content of thallium element in the Via-filling in (3) and (4) above should be relatively close to zero compared to the content of bismuth element. The ratio of both catalyst elements (thallium element content/bismuth element content) in the gold deposit should be at least ⅓, preferably 1/10 or greater, more preferably 1/100 or greater, or even more preferably 1/1,000 or greater.
In any of (1) to (4), regarding (d) the bismuth catalysts, its lower limit is 30 mg/L, and 35 mg/L is preferred. The upper limit of the bismuth element is 150 mg/L, and 140 mg/L is preferred. In any of (1) to (4), regarding (e) the thallium catalysts, its lower limit is 5 mg/L, and 6 mg/L is preferred. Its upper limit is 50 mg/L, and 45 mg/L is preferred. The upper and lower limits of the bismuth and thallium elements are determined to secure that the bismuth element in the gold deposit becomes in the range of 0.01 to 100 ppm.
It is also preferred that said (d) bismuth catalyst is one or more of bismuth nitrate, ammonium bismuth citrate and bismuth sulfamate, and said (e) thallium catalyst is one or more of thallium formate, thallium malonate and thallium nitrate. It is particularly preferred that said (d) bismuth catalyst is bismuth nitrate and said (e) thallium catalyst is thallium formate.
The inventions of the gold electroplating solution according to any of the above (1) through (8) are all exemplified by the invention of a gold electroplating method in which a gold electroplating solution based on gold (I) sulfite complex, sulfate and sulfite or sulfite is co-added with trace amounts of bismuth and thallium compounds to electroplate the inside of the drilled hole to obtain a U-shaped stacked structure, as seen in cross section.
In addition, any of the inventions of gold electroplating solution according to any of the above (1) to (8) discloses the invention of a gold electroplating method for densely Via-filling the inside of a number of drilled holes using this gold electroplating solution. In addition, any of the inventions of gold electroplating solution according to any of the above (1) to (8) discloses an invention of a gold electroplating method in which bismuth element is more involved than thallium element in the gold deposit using this gold electroplating solution.
Furthermore, any of the inventions of gold electroplating solution according to any of the above (1) to (8) discloses the invention of a flat gold plating layer of gold deposit containing only bismuth preferentially over thallium, which is formed by this gold electroplating solution. Moreover, any of the inventions of gold electroplating solution according to (1) to (8) above all disclose the invention of a U-shaped layered structure in cross section filled with dense Via-filling formed inside a drilled hole by this gold electroplating solution. Furthermore, any of the inventions of gold electroplating solution according to (1) through (8) above also includes providing a method for manufacturing the above gold electroplating solution.
For example, the gold electroplating solution of (1) above discloses the invention of a gold electroplating method for Via-filling the inside of a drilled hole in a U-shape using the gold electroplating solution of (1) above, as shown in the examples, etc. described below. Also disclosed is the invention of a flat gold plating layer of gold electrodeposits formed by using the gold electroplating solution of (1) above, in which more than 0.01 ppm of the bismuth element is entrained and less than 0.1 ppm or preferably less than 0.01 ppm of the thallium element is entrained. These disclosed inventions of gold electroplating methods constitute herein an invention separate from the invention of gold electroplating solution.
In particular, the invention of the following gold electroplating method in which a direct current is applied is disclosed in the invention of the gold electroplating solution.
The following embodiments also form part of the gold electroplating method according to the present invention.
The gold electroplating solution of the present invention is able to completely and defect-freely Via-fill drilled holes with electrodeposit, even inside high aspect ratio drilled holes where the current flow does not reach. Moreover, as a result of the adjustment of the content of the bismuth and thallium elements in the gold electroplating solution, the gold deposit electroplated by the electrolytic deposition reaction has the unique phenomenon of containing no thallium element, which is harmful to the human body, although bismuth is preferentially contained in the electroplated gold deposit. Thallium elements are highly toxic and no more than 0.1 ppm of them should be detected in the gold plating layer in printed wiring circuits.
When the cross section of the drilled hole was observed, it was found that the gold electroplating solution of the present invention deposits gold electrodeposit which has a U-shaped cross section. In high-aspect-ratio drilled holes, a U-shaped structure with a hemispherical cross section is compactly deposited from the center of the aperture. The gold electroplating solution of the present invention has the specific effect of causing the depositing gold deposits to have a U-shape in cross section even in a deeper section of the drilled hole where the current density distribution is the sparser. In other words, the gold electroplating solution of the present invention can constantly create a compact Via-filling having a U-shaped cross section even at such depths of a drilled hole where the current flow fails to reach.
The gold electroplating solution of the present invention is an improvement of gold electroplating solution with two types of metal additives. It is known that the effect of metal additives to gold electroplating solution is to provide excellent liquid stability and to help obtain uniform plating thickness on the flat circuit surface of the plated object, even with wide and narrow complex line widths. The gold electroplating solution of the present invention also possesses these known effects.
However, the specific effect of this invention has not been known with regard to conventional gold electroplating solutions containing metal additives. Namely, conventional gold electroplating solutions could not help produce compactly packed Via-fill plating in places where the current flow was insufficient, such as at B of
Furthermore, the above effects of the gold electroplating solution of the present invention extend to the invention of a gold electroplating method for electroplating the inside of drilled holes by adding trace amounts of thallium and bismuth compounds to a gold electroplating solution based on gold (I) sulfite complex, sulfate and sulfurous acid or sulfite. The above effects of the gold electroplating solution also extend to the invention of a gold electroplating method for Via-filling inside a number of drilled holes using the gold electroplating solution of the present invention.
The above effects of the gold electroplating solution of the present invention also extend to the invention of the method of manufacturing the gold electroplating solution of the present invention. In other words, the gold electroplating solution of the present invention can achieve the above effects by manufacturing a gold electroplating solution having the prescribed composition of components. The specific effects of the gold electroplating solution of the present invention also extend to the invention of dense Via-filling deposit with a U-shaped stacked structure in cross section inside drilled holes, which are filled inside a number of drilled holes using the gold electroplating solution of the present invention, and to the invention of gold deposits containing bismuth in preference to thallium.
The gold electroplating method according to the present invention has the effect that even if the aspect ratio of the drilled hole differs and the current density distribution on the deposition surface on the plated object changes, a compact Via-filling with a U-shaped cross section is always formed inside the drilled hole by virtue of the coordinated adsorption/desorption reaction by both catalysts, as shown in
Furthermore, according to the gold electroplating method of the present invention, the grain structure of the compactly-packed deposit inside the drilled hole has a metallurgical thermal recovery effect. This does not require any special heat treatment. The gold purity of gold deposits produced by electroplating operations using the gold electroplating solution is 99.99% or higher. This is because such pure gold electrodeposits are metallurgically self-healing by thermal energy, such as in subsequent gold plating operations.
According to the gold electroplating method of the present invention, the following effects are achieved in addition to the effects of the gold electroplating solution of the present invention described above.
In other words, the gold electroplating method of the present invention is effective in densely filling the inside of through-silicon wafer vias (TSV) and blind via holes of copper-clad laminates with gold deposits by simply applying an ordinary DC current without using a complicated pulse power supply. Via-filling of TSVs and blind via-holes in copper clad laminates with gold deposits is possible. In addition, the gold electroplating method of the present invention has the effect that long-term stable gold electroplating work can be continuously performed by simply replenishing consumed sulfurous acid or sulfite and gold sulfite, without replenishing bismuth catalyst and thallium catalyst. The gold electroplating method also has the effect that the catalyst suppression reaction in the gold electroplating solution does not fluctuate even if the Via-filling is repeated many times.
Described hereinunder is the description of each embodiment of the invention. The gold electroplating solution of one embodiment of the present invention is a gold electroplating solution containing mainly gold ions derived from gold (I) sulfite complexes, predetermined amounts of bismuth and thallium catalysts, and sulfite ions in a specified ratio. The gold electroplating method of one embodiment of the present invention is a gold electroplating method for Via-filling the interior of a drilled hole using a gold electroplating solution chiefly comprising the sulfite ions and such predetermined amounts of bismuth catalyst and thallium catalyst that they cause desirable coordinated adsorption and desorption reaction.
The invention will now be illustrated in drawings.
As shown in
The drawing on the right side of
By varying the concentration of the bismuth and thallium elements in the gold electroplating solution, it is possible to make the thallium component be detected relatively more in the electrolytically deposited gold plating layer than the bismuth component. For example, if the concentration of the bismuth element is relatively thin or the concentration of the thallium element is relatively high, the thallium catalyst is preferentially adsorbed and desorbed, and the thallium component is easily entrained in the gold deposit.
As the Via-filling by the present invention progresses, the space in the perforation hole becomes shallower and the current density distribution inside and above the perforation hole becomes equal, and the bismuth catalyst becomes less active. Then, the existing electrodeposition reaction will dominate instead of the cooperative adsorption/desorption reaction. Precise measurement of the thickness (per unit time) of the Via-filled gold deposit in the case of (b) of
In this document, thallium ions (metal) and bismuth ions (metal) in the cooperative adsorption/desorption and catalytic deposition reactions of thallium ions (metal) and bismuth ions (metal) in gold electroplating solutions and methods, or in electrolytic deposition reactions, are herein conveniently called “thallium catalyst” and Bismuth catalyst”. In other words, in the gold plating solution according to the present invention, a certain amount of bismuth catalyst and thallium catalyst not only act as a crystal modifier in the electrodeposition reaction where the cathodic current density is high, but also show a catalytic inhibition reaction of thallium catalyst. The bismuth and thallium catalysts have a cooperative catalytic action that repeatedly adsorbs and desorbs to the plated object surface inside the perforated hole through a barrier layer of sulfite ion groups in the catalytic deposition reaction with weak cathodic current density. In this document, for the sake of convenience, the term “electrodeposition reaction” is used at the perimeter of the perforated hole shown in A of
“Via-filling” is a plating technique in which the interior of the drilled hole is filled with a layered, stacked structure. This is especially the bottom of the drilled hole and its periphery. The gold electroplating solution of the present invention can preferentially deposit and fill plated areas such as these through a coordinated adsorption and desorption reaction.
By using the gold electroplating solution of the present invention, it is possible to obtain Via-filling deposits that are compactly-packed inside plating of the drilled holes. In the present invention, the cross-sectional shape of the densely filled Via-filling deposit is not limited to rectangular. It can be barrel-shaped or inverted barrel-shaped, trapezoidal or inverted trapezoidal. This is because even the shape of the drilled hole, which normally does not allow current flow, can be densely filled by the catalytic inhibition reaction of the present invention.
Oxygen from the atmosphere is taken into the gold electroplating solution before the electroplating process starts, and it exists as dissolved oxygen in the gold electroplating solution. When the dissolved oxygen reacts with the gold (I) sulfite complex, the gold complex is decomposed and gold particles are deposited. However, when excess sulfite ions coexist in the gold electroplating solution, the dissolved oxygen reacts with the sulfite ions before the gold (I) sulfite complex to form sulfate ions. In the gold electroplating solution, bismuth ions (Bi3+) and thallium ions (Tl+) also act autonomously as existing crystal conditioners. As a result, the gold (I) sulfite complex can exist stably in the gold electroplating solution without decomposition. In other words, the gold electroplating solution of the present invention is liquid stable. The gold electroplating solution of the present invention is stable before and after electroplating operations.
The gold electroplating solution of the present invention can form a compactly packed Via-filling with a U-shaped stacked structure in the cross section inside the drilled hole where the current flow is weak, and even if the current density distribution of the plated material is coarse or dense, the catalyst suppression reaction and electrolytic deposition reaction can switch over autonomously. In the gold electroplating solution of the present invention, bismuth ion (Bi3+) and thallium ion (Tl+) also have the effect of acting autonomously as catalysts or existing crystal conditioners.
The content of gold (I) sulfite complex in gold electroplating solution for the present invention can be determined according to the electroplating workload, as with conventional gold electroplating solutions. Gold (I) sulfite complexes can be made from one or more gold (I) sulfite alkali metals, such as gold (I) sodium sulfite, gold (I) potassium sulfite, gold (I) ammonium sulfite, gold (I) ethylammonium sulfite, gold (I) dimethylammonium sulfite, gold (I) diethylammonium sulfite, gold (I) trimethylammonium sulfite, or gold (I) triethylammonium sulfite.
In the gold electroplating solution and gold electroplating method for the present invention, sulfite or sulfite is a group of sulfite ions that stabilize the gold electroplating solution. In other words, aqueous solutions containing sulfite or sulfite protect gold (I) sulfite complexes by converting dissolved oxygen into sulfate ions. Dissolved oxygen generated during the electroplating process is also converted to sulfate ions. Therefore, the gold sulfite (I) complex in the gold electroplating solution does not decompose and a stable gold complex gold electroplating solution can be obtained. In addition, an increase in the concentration of sulfurous acid or sulfite has the effect of accelerating the deposition rate of gold electrodeposits inside the drilled holes.
The present invention requires 5 to 200 g/L of sulfite. If the sulfite is less than 5 g/L, the gold (I) sulfite complex decomposes in the gold electroplating solution and colloidal gold particles are easily formed. When sulfite exceeds 200 g/L, colloidal gold particles are more likely to form. This is because dithionite ions may be formed. The preferred content of sulfite is 20 to 150 g/L. The more preferred content of sulfite is from 30 to 100 g/L. The particularly preferred content of sulfite is 40-60 g/L.
In addition, 3 to 150 g/L of sulfate is required in the present invention. Sulfate is necessary to stabilize the sulfite ion group in the gold electroplating solution. If the sulfate content is less than 3 g/L, the sulfite group tends to decompose during gold electroplating, making it difficult for the sulfite group to act as a barrier agent. If the sulfate content exceeds 150 g/L, sulfate crystals may precipitate in the gold electroplating solution. The preferred content of sulfate is 5 to 100 g/L. The more preferred content of sulfate is 10 to 50 g/L. Particularly preferred sulfate content is 15 to 30 g/L.
Bismuth catalysts in the gold electroplating solution and gold electroplating method can be selected from known ones useful for gold electroplating solution. Such include but not limited to bismuth alkane sulfonate salts, e.g., bismuth methanesulfonate, bismuth ethanesulfonate, bismuth propanesulfonate, 2-bismuth propanesulfonate, and bismuth p-phenol sulfonate, Bismuth alkanol sulfonate salts, e.g., bismuth hydroxymethanesulfonate, bismuth 2-hydroxyethane-1-sulfonate, and bismuth 2-hydroxybutane-1-sulfonate, bismuth gluconate, and bismuth lactate, as well as inorganic bismuth salts, such as bismuth oxide, bismuth hydroxide, bismuth carbonate, bismuth trifluoride, bismuth bromide, bismuth nitrate, bismuth sulfate, bismuth pyrophosphate and bismuth chloride. Water-soluble bismuth compounds (e.g., aqueous bismuth hydrochloride, bismuth nitrate, bismuth sulfamate, bismuth phosphate, bismuth diphosphate, bismuth acetate, bismuth citrate, bismuth phosphonate, bismuth carbonate, bismuth oxide, bismuth hydroxide) are preferred bismuth catalysts. Bismuth nitrate, bismuth ammonium citrate and bismuth sulfamate are particularly preferred.
The content of bismuth catalyst must be 30 to 150 mg/L in terms of bismuth element. This range prevents the thallium element from entering the gold deposit. At least 30 mg/L is required as the lower limit of the bismuth element, and 35 mg/L is preferred. The upper limit of the bismuth element is 150 mg/L, and 140 mg/L is allowable. In other words, the preferred content of bismuth catalyst is 35 to 140 mg/L.
Thallium catalysts required for the gold electroplating solution and gold electroplating method of the present invention include sulfates, acetates, nitrates, sulfides, chlorides, borosilicates, and other organic salts. The thallium catalyst can be either primary or secondary thallium as long as it is a soluble salt. The thallium catalyst is not entrapped in the gold deposit in the gold electroplating solution according to the composition of the components of the present invention. Preferred thallium catalysts are thallium formate, thallium sulfate, thallium nitrate, thallium carbonate, thallium oxide, thallium bromide, thallium acetate, and thallium malonate. Thallium formate, thallium malonate and thallium nitrate are especially preferred.
The thallium catalyst content requires 5-50 mg/L of thallium element. If the thallium catalyst content exceeds 50 mg/L, the thallium element may be included in the gold deposit due to coordination with the bismuth catalyst content. The upper limit of thallium element is 50 mg/L, and 45 mg/L is preferred. If the lower limit of thallium catalyst is less than 5 mg/L, it may be difficult to deposit fine gold deposits inside the depressions. The lower limit of thallium element is 5 mg/L, and 6 mg/L is preferred. In other words, the preferred content of thallium catalyst is 6-45 mg/L.
The weight ratio of the bismuth catalyst to the thallium catalyst (bismuth element/thallium element) of 0.6 to 30 is preferred. Within this ratio range, the catalytic inhibition reaction from gold (I) sulfite complex ([Au(SO3)2−]3−) to gold (Au) is further promoted by the following reaction equations (1) and (2).
Au(SO3)23−→Au++2SO32− (1)
Au++e→Au (2)
A lower limit of 0.6 for the ratio of bismuth catalyst to the thallium catalyst may cause coarse gold to deposit inside the concaves. If this ratio exceeds the upper limit of 30, voids and cavities may form on the walls and inside of the concaves. The ratio of the bismuth catalyst to the thallium catalyst is preferably greater than 1 but 30 or less. A more preferred ratio is from 3 to 25, even more preferably from 5.0 to 20, and a particularly preferred ratio is from 7.0 to 15. The weight ratio of the total amount of the above bismuth catalyst (D) and the above thallium catalyst (C) to the weight of the above sulfite or sulfite (A) was examined, and the range of 1.4×10−4 to 400×10−4 is preferred.
Furthermore, the conducting salts other than sulfates and sulfurous acid or sulfites (collectively referred to as “interferents”) required for the gold electroplating solution and gold electroplating method of the present invention are electro-conducting salts and complexing agents. Typical examples are additives commonly added to gold electroplating solutions. These conducting salts can also include pH adjusters (buffers) and masking agents. Conducting salts can be added to the extent that they do not interfere with the formation of the cross-sectional U-shaped deposition structure.
One type of conducting salt may be used alone or in combination with two or more types. Possible conducting salts include inorganic salts such as halides, nitrates, carbonates, and phosphates, and organic salts such as acetates, oxalates, citrates, and carboxylates. Preferable choices are halide, nitrate, carbonate, phosphate, acetate, oxalate or citrate. More preferably carbonate, phosphate, acetate or oxalate.
Conducting salts improve the coarseness of the current density distribution in gold electroplating solution. Therefore, the addition of conducting salts to the gold electroplating solution of the present invention promotes the normal electrolytic deposition reaction in gold electrodeposits on the one hand, and inhibits the catalytic inhibition reaction on the other hand. However, if too much conductive salt is added, as shown at (a) of
The pH range in the gold electroplating solution of the present invention is preferably between 6 and 13. The gold (I) sulfite complexes tend to become unstable when the pH is below 6. On the other hand, if the pH exceeds 13, masking agents such as photoresist may be dissolved. pH in the range of 7 to 12 is more preferable.
The current density in the gold electroplating method of the present invention is preferably in the range of 0.03-0.6 A/m2 using DC current; if the current density exceeds 0.6 A/m2, cavities are likely to form inside the drilled holes. If the current density is less than 0.03 A/m2, the interior of the drilled hole may not be electroplated. In the case of jet stream plating, the current density should be in the range of 0.1 to 1.0 A/dm2. Jet stream plating method is highly desirable for mass production.
In the gold electroplating solution of the present invention and gold electroplating method thereof, the plated object can be a metal-coated wiring circuit such as semiconductor wafers, ceramic wafers and printed circuit boards. Typical semiconductor and ceramic wafers are Si and GaAs substrates. Printed circuit boards can be copper clad laminates, etc. The underlying metal coating inside the drilled holes preferably consists of a gold film. The drilled holes preferably have an opening area of 1 to 50 μm in terms of the diameter of the opening, and an aspect ratio of 0.8 to 2.0.
The following examples will illustrate the invention in detail. Gold electroplating solutions (01) to (08) were prepared with the compositions described below. These eight gold electroplating solutions correspond to Examples 1-8.
The gold electroplating solution (01) in Example 1 contained the following compositions A through F, while pH=8.0.
Here, the weight ratio of both catalysts (D: thallium formate, and E: bismuth nitrate combined) to the barrier agent (B: sodium sulfite), i.e., both catalysts (Bi+Tl)/barrier agent (Na2SO3), was 12×10−4. The weight percentage of the conductive salt (F: sodium phosphate) to the interfering agent (the sum of B: sodium sulfite and C: sodium sulfate), i.e., conductive salt (Na3PO4)/interfering agent (Na2SO3+Na2SO4) was 1.5%. The plating solution was stable before and after the electroplating operation, and no gold precipitates were observed after the electroplating operation was completed.
The gold electroplating solution (02) in Example 2 contained the composition of components A through F below, while pH=12.0.
Here, the weight ratio of both catalysts (D: thallium formate and E: bismuth nitrate combined) to the barrier agent (B: sodium sulfite), i.e., both catalysts (Bi+Tl)/barrier agent (Na2SO3) is 14×10−4. The weight ratio of thallium catalyst to bismuth catalyst, i.e., (Bi)/(Tl), is 3.5. The weight ratio of the conducting salt (F: sodium nitrate) to the disturbing agent (the sum of B: sodium sulfite and C: sodium sulfate), i.e., conducting salt (NaNO3)/disturbing agent (Na2SO3+Na2SO4), is 5.3%. This plating solution was also stable before and after the electroplating process, and no gold precipitates were observed after the electroplating process was completed.
The gold electroplating solution (03) in Example 3 contained the composition of components A to F below, while pH=7.0.
Here, the weight ratio of both catalysts (the sum of D: thallium malonate and E: ammonium bismuth citrate) to the barrier agent (B: sodium sulfite), or both catalysts (Bi+Tl)/barrier agent (Na2SO3) was 146×10−4. The weight ratio of thallium catalyst to bismuth catalyst, or (Bi)/(Tl), was 23.3. The plating solution was also stable before and after the electroplating process, and no gold precipitates were observed after the electroplating process was completed.
The gold electroplating solution (04) in Example 4 contained the composition of components A-F below, while pH=10.0.
Here, the weight ratio of both catalysts (D: thallium nitrate and E: bismuth sulfamate combined) to the barrier agent (B: sodium sulfite), i.e., both catalysts (Bi+Tl)/barrier agent (Na2SO3) was 4.4×10−4. The weight ratio of thallium catalyst to bismuth catalyst, or (Bi)/(Tl), was 0.8. The plating solution was also stable before and after the electroplating process, and no gold precipitates were observed after the electroplating process was completed.
The gold electroplating solution (05) in Example 5 contained the composition of components A to F below, while pH=8.0.
Here, the weight ratio of both catalysts (D: thallium nitrate and E: bismuth sulfamate combined) to the barrier agent (B: sodium sulfite), i.e., both catalysts (Bi+Tl)/barrier agent (Na2SO3) was 5.4×10−4. The weight ratio of thallium catalyst to bismuth catalyst, or (Bi)/(Tl), was 6.0. The plating solution was also stable before and after the electroplating process, and no gold precipitates were observed after the electroplating process was completed.
The gold electroplating solution (06) in Example 6 contained the composition of components A to F below, while pH=8.0.
Here, the weight ratio of both catalysts (D: thallium nitrate and E: bismuth sulfamate combined) to the barrier agent (B: sodium sulfite), i.e., both catalysts (Bi+Tl)/barrier agent (Na2SO3) was 30.8×10−4. The weight ratio of thallium catalyst to bismuth catalyst, or (Bi)/(Tl), was 3.0. The plating solution was also stable before and after the electroplating process, and no gold precipitates were observed after the electroplating process was completed.
The gold electroplating solution (07) of Example 7 contained the composition of components A to F below, while pH=8.0.
and
Here, the weight ratio of both catalysts (the sum of D: thallium nitrate and E: bismuth nitrate) to the barrier agent (B: sodium sulfite), i.e., both catalysts (Bi+Tl)/barrier agent (Na2SO3) was 9.2×10−4. The weight ratio of thallium catalyst to bismuth catalyst, or (Bi)/(Tl), was 5.0. The plating solution was also stable before and after the electroplating process, and no gold precipitates were observed after the electroplating process was completed.
The gold electroplating solution (08) of Example 1 contained the composition of components A to F below, while pH=8.0.
and
Here, the weight ratio of both catalysts (D: thallium formate and E: bismuth nitrate combined) to the barrier agent (B: sodium sulfite), i.e., both catalysts (Bi+Tl)/barrier agent (Na2SO3), was 9.2×10−4. The weight ratio of thallium catalyst to bismuth catalyst, or (Bi)/(Tl), was 5.0. The plating solution was also stable before and after the electroplating process, and no gold precipitates were observed after the electroplating process was completed.
The component compositions and weight percentages, etc. of each component for the gold electroplating solutions (01) to (08) in Examples 1 to 8 above are listed in a simplified form in Table 1.
A conductor circuit pattern was fabricated on the entire surface of a 3.0 mm thick silicon wafer substrate (200 mm diameter). Next, the following (Va)-(Vc) perforation holes (aspect ratio: Va-Vc) were drilled in this substrate, and then a 0.3 μm titanium-tungsten alloy intermediate film was vacuum deposited on this substrate, followed by a 0.1 μm gold base film. This was used as test substrate 1. A base gold film was also formed inside the drilled holes of test substrate 1.
Aspect ratio: Va (diameter: 10 μm; depth: 10 μm; pitch: 50 μm) 10 pcs.
Aspect ratio: Vb (diameter: 5 μm, depth: 10 μm, pitch: 70 μm) 10 pcs.
Aspect ratio: Vc (diameter: 3 μm, depth: 9 μm, pitch: 100 μm) 10 pcs.
Test substrate 2 was prepared in the same way as test substrate 1, except that instead of 0.1 μm vacuum-deposited gold, 0.1 μm vacuum-deposited palladium was used. The inside of the drilled holes (aspect ratio: Va-Vc) in the test substrate 2 was also covered with the underlying palladium film.
A gold conductor circuit pattern was fabricated on the entire surface of a 3.0 mm thick silicon wafer substrate (200 mm diameter). First, 80 perforation holes of the following (Vc) were drilled in this substrate, and then a 0.3 μm titanium-tungsten alloy intermediate film was vacuum deposited, followed by a 0.1 μm gold coating film on top of them. This was used as test substrate 3. The inside of the drilled holes in test substrate 3 is also covered with the underlying gold vapor deposition film.
Aspect ratio: Vc (diameter: 3 μm, depth: 6 μm, pitch: 100 μm)
Palladium conductor circuit patterns were fabricated on the entire surface of a 3.0 mm thick silicon wafer substrate (200 mm diameter). Then, 80 drilled holes of the following (Vc) were drilled in this substrate, followed by a vacuum evaporation deposition of a 0.3 μm thick titanium-tungsten alloy intermediate film and a vacuum evaporation deposition of a 0.1 μm thick palladium coating film. This was used as test substrate 4.
Aspect ratio: Vc (diameter: 3 μm; depth: 6 μm; pitch: 100 μm)
Next, using the above gold electroplating solutions (01) through (08) and test substrates 1 through 4, various characteristics were evaluated by the following examples. The results of these evaluations are described below.
The cathodic polarization curves of potential (vs. standard potential (Ag/AgCl)) and current in the gold electroplating solution (01) of Example 1 were measured using an Electro Chemical Measurement System (model: HZ-7000) manufactured by HOKUTO DENKO. This is Example 9. The cathodic polarization curve of Example 9 is shown by the solid line in
Using gold electroplating solution (01) at a solution temperature of 55° C., a DC current was applied to test substrate 1 to thereby effect gold electroplating. The electroplating operation was performed for 4 minutes. Test substrate 1 was then rinsed and dried. This was Example 10. Cross-sectional photographs of the interior of the drilled holes of Example 10 are shown in
The cross section of the Via-filling filled with gold deposit inside the drilled hole (diameter: 10 μm, depth: 10 μm) of Va in
It can also be seen that the crystal grains of the gold plating film at the periphery and inside of the drilled hole are different. Comparing the black grains, those at both ends (periphery) of Va of
The gold deposit inside the drilled hole (diameter: 5 μm, depth: 10 μm) in Vb of
The gold deposit inside the drilled hole (diameter: 3 μm; depth: 9 μm) in Vc of
Comparing Va and Vb in
The aspect ratio in Vc of
Test substrate 1 was gold electroplated for 8 minutes using gold electroplating solution (01) in the same manner as in Example 10. This is Example 11 and a photograph of the cross section after the 8 minutes is shown in
The gold deposit cross sections inside the drilled holes in the gold plating films of Va, Vb, and Vc in
Next,
Test substrate 3 was plated in gold electroplating solution (01) with cathodic current densities of 0.2 A/dm2 and 0.4 A/dm2 to thereby obtain Via-filling deposition inside the drilled holes. The gold electroplating solution (01) contained 10 mg/L of thallium formate (as thallium element) and 50 mg/L of bismuth nitrate (as bismuth element). Subsequently, a glow discharge mass spectrometer (model: Astrum) manufactured by AMETEK was used to analyze trace amounts of bismuth and thallium elements in the Via-filled gold deposits.
The bismuth element in the deposits obtained by the gold electroplating process measured 1.24 ppm and 1.78 ppm at cathode current densities of 0.2 A/dm2 and 0.4 A/dm2, respectively. On the other hand, the thallium element failed to be detected when measured by an analyzer capable of detecting thallium at an order of 0.01 ppm ( manufactured by). When the similar Au-electroplating was performed with a 5° C. lower solution temperature, the bismuth element content increased, but none of the thallium elements were detected.
The analysis results in Example 12 show that, in the case of gold electroplating solution of the present invention, the bismuth element in the gold deposit increases with increasing cathode current density. On the other hand, the thallium element is not included in the gold deposit. The fact that the content of the bismuth element is also affected by the current density and liquid temperature indicates that if plating conditions suitable for the aspect ratio of the drilled holes are selected, it is possible to minimize the entrainment of the bismuth element in the gold deposit.
Next, test substrate 1 and test substrate 2 were gold electroplated for 4 and 8 minutes using gold electroplating solutions (02) through (08). The cross sections inside the drilled holes of test substrate 1 and test substrate 2 after 4 and 8 minutes were observed for each gold electroplating solution (02) to (08), respectively. These cross-sectional photographs (not shown) were similar to the cross-sectional photographs of the drilled holes (Va, Vb and Vc) in
Next, a conductive circuit pattern with a gold film was fabricated on the entire surface of a 3.0 mm thick silicon wafer substrate (200 mm diameter) to make Test Substrate 3 in the examples. The test substrate 3 was electroplated by applying DC current to the gold electroplating solutions (01) to (06) of the examples, and electroplating operation was performed at a liquid temperature of 55° C. for 8 minutes. That is, the plating film in Example 41 was obtained by gold electroplating solution (01), Example 42 was from gold electroplating solution (02), Example 43 was from gold electroplating solution (03), Example 44 was from gold electroplating solution (04), Example 45 was from gold electroplating solution (05), and Example 46 was from gold electroplating solution (06).
The thickness of the gold plating film on test substrate 3 was sectioned by focused ion beam (FIB). Five cross sections were measured with a SIM device (Hitachi High-Technologies, MI4050), and the mean and standard deviation of the five cross sections were determined. The results of the film thickness of the six gold-plated films in Examples 41-46 are shown in Table 2.
As shown in Table 2, there was little difference in the average thickness and its deviation of the gold-plated film thickness for test substrates 3 in Examples 41-46. In other words, the average film thicknesses of the gold electroplating solutions (01) to (06) in Examples 41 to 46 ranged narrowly from 2.11 to 2.18 μm, and the standard deviation (3σ) ranged no more than from 0.12 to 0.15.
Next, a palladium film conductor circuit pattern was fabricated on the entire surface of a 3.0 mm thick silicon wafer substrate (200 mm diameter) to make test substrate 4 in the examples. The test substrate 4 was subjected to the same electroplating operation as test substrate 3, applying DC current to the gold electroplating solutions (01) through (06) in the examples and electroplating at a liquid temperature of 55° C. for 8 minutes, and the average values and standard deviations for the five locations were determined. The results of the film thickness of the six gold-plated films in Examples 47-52 are shown in Table 3.
As Table 3 shows, there were only slight differences in the average thickness and its deviation of the palladium plating film thickness for Test Substrates 4 in Examples 47-52. In other words, the average film thicknesses of the gold electroplating solutions (01) to (06) in Examples 47 to 52 ranged only from 2.13 to 2.25 μm, and the standard deviation (3σ) ranged scarcely from 0.11 to 0.14.
Next, comparative examples are described and specifically compared to the examples of the present invention. Gold electroplating solutions (09) to (18) with the following compositions were prepared as comparative examples. These gold electroplating solutions were designated as Comparative Examples 1 to 10.
Gold electroplating solution (09) contained the following composition of components A to F, while pH=8.0.
Gold electroplating solution (09) is the same as gold electroplating solution (01) except that thallium formate was excluded (specified as “0 mg/L”). Here, the weight ratio of both metal salts (D: thallium formate and E: bismuth nitrate combined) to the barrier agent (B: sodium sulfite), i.e., sum of metal salts (Bi+Tl) divided by barrier agent (Na2SO3) was 10×10−4. The plating solution was stable before and after the electroplating process, and no gold precipitates were observed after the electroplating process had been completed.
Gold electroplating solution (10) contained the following composition of components A to F, while pH=8.0.
Gold electroplating solution (10) was the same as gold electroplating solution (01) except that bismuth nitrate was excluded (specified as “0 mg/L”). Here, the weight ratio of both metal salts (D: thallium formate plus E: bismuth nitrate combined) to the barrier agent (B: sodium sulfite), i.e., sum of metal salts (Bi+Tl) divided by barrier agent (Na2SO3) was 2.0×10−4. The weight ratio of thallium catalyst to bismuth catalyst, i.e., (Bi)/(Tl), was 0. This plating solution was also stable before and after the electroplating process, and no gold precipitates were observed after the electroplating process had been completed.
Gold electroplating solution (11) contained the following composition of components A to F, while pH=6.0.
Gold electroplating solution (11) in Comparative Example 3 was same as gold electroplating solution (01) except that D: thallium formate was 60 mg/L, E: bismuth nitrate was 180 mg/L, F: ammonium phosphate was 6 g/L and pH=6.0. Here, the weight ratio of both metal salts (D: thallium formate and E: bismuth nitrate combined) to the barrier agent (B: sodium sulfite), i.e., metal salts combined (Bi+Tl)/barrier agent (Na2SO3) was 48×10−4. The weight ratio of thallium salt to bismuth salt, or (Bi)/(Tl), was 3.0. This plating solution was also stable before and after the electroplating process, and no gold precipitates were observed after the electroplating process had been completed.
Gold electroplating solution (12) contained the following composition of components A to F, while pH=8.0.
The gold electroplating solution (12) of Comparative Example 4 is the same as the gold electroplating solution (01) except that D: thallium formate was 30 mg/L and E: bismuth nitrate was 10 mg/L. Here, the weight ratio of both metal salts (the sum of D: thallium formate and E: bismuth nitrate) to the barrier agent (B: sodium sulfite), i.e., both metal salts (Bi+Tl)/barrier agent (Na2SO3) was 8×10−4. The weight ratio of thallium salt to bismuth salt, i.e., (Bi)/(Tl) was 3.0. The plating solution was also stable before and after the electroplating process, and no gold precipitates were observed after the electroplating process had been completed.
The gold electroplating solution (13) contained the following composition of ingredients A to F, while pH=8.0.
Gold electroplating solution (13) is the same as gold electroplating solution (05) except that D: thallium formate was 3 mg/L and E: bismuth nitrate was 70 mg/L. Here, the weight ratio of both metal salts (the sum of D: thallium formate and E: bismuth nitrate) to the barrier agent (B: sodium sulfite), that is, both metal salts (Bi+Tl)/barrier agent (Na2SO3) was 11.2×10−4. The weight ratio of thallium salt to bismuth salt, i.e., (Bi)/(Tl) was 3.0. The plating solution was also stable before and after the electroplating process, and no gold precipitates were observed after the electroplating process had been completed.
Gold electroplating solution (14) contained the following composition of components A to F, while pH=8.0.
Gold electroplating solution (14) is the same as gold electroplating solution (05) except that D: thallium formate was 60 mg/L and E: bismuth nitrate was 70 mg/L. Here, the weight ratio of both metal salts (the sum of D: thallium formate and E: bismuth nitrate) to the barrier agent (B: sodium sulfite), i.e., both metal salts (Bi+Tl)/barrier agent (Na2SO3) was 20×10−4. The weight ratio of thallium salt to bismuth salt, i.e., (Bi)/(Tl) was 1.2. The plating solution was also stable before and after the electroplating process, and no gold precipitates were observed after the electroplating process had been completed.
Gold electroplating solution (15) contained the following composition of components A to F, while pH=8.0.
Gold electroplating solution (15) is the same as gold electroplating solution (05) except that D: thallium formate was 20 mg/L and E: bismuth nitrate was 20 mg/L. Here, the weight ratio of both metal salts (the sum of D: thallium formate and E: bismuth nitrate) to the barrier agent (B: sodium sulfite), that is, both metal salts (Bi+Tl)/barrier agent (Na2SO3) was 6.2×10−4. The weight ratio of thallium salt to bismuth salt, i.e., (Bi)/(Tl) was 1. The plating solution was also stable before and after the electroplating process, and no gold precipitates were observed after the electroplating process had been completed.
Gold electroplating solution (16) contained the following composition of components A to F, while pH=8.0.
Gold electroplating solution (16) is the same as gold electroplating solution (05) except that D: thallium formate was 20 mg/L and E: bismuth nitrate was 180 mg/L. Here, the weight ratio of both metal salts (the sum of D: thallium formate and E: bismuth nitrate) to the barrier agent (B: sodium sulfite), that is, both metal salts (Bi+Tl)/barrier agent (Na2SO3) was 6.2×10−4. The weight ratio of thallium salt to bismuth salt, i.e., (Bi)/(Tl) was 9. The plating solution was also stable before and after the electroplating process, and no gold precipitates were observed after the electroplating process had been completed.
Gold electroplating solution (17) contained the following composition of components A to F, while pH=8.0.
Gold electroplating solution (17) is the same as gold electroplating solution (07) except that D: thallium nitrate was 30 mg/L and E: bismuth nitrate was 10 mg/L. Here, the weight ratio of both metal salts (the sum of D: thallium formate and E: bismuth nitrate) to the barrier agent (B: sodium sulfite), i.e., both metal salts (Bi+Tl)/barrier agent (Na2SO3) was 6.2×10−4. The weight ratio of thallium salt to bismuth salt, or (Bi)/(Tl), was 0.3. The plating solution was also stable before and after the electroplating process, and no gold precipitates were observed after the electroplating process had been completed.
Gold electroplating solution (18) contained the following composition of components A to F, while pH=8.0.
Gold electroplating solution (18) is the same as gold electroplating solution (08) except that D: thallium formate was 30 mg/L and E: bismuth nitrate was 10 mg/L. Here, the weight ratio of both metal salts (the sum of D: thallium formate and E: bismuth nitrate) to the barrier agent (B: sodium sulfite), i.e., both metal salts (Bi+Tl)/barrier agent (Na2SO3) was 6.2×10−4. The weight ratio of thallium salt to bismuth salt, i.e., (Bi)/(Tl) was 0.3. The plating solution was also stable before and after the electroplating process, and no gold precipitates were observed after the electroplating process had been completed.
The component compositions and weight percentages, etc. of each component of the gold electroplating solutions (09) to (18) in Comparative Examples 1 to 10 are listed in Table 4.
Next, the potential (pairwise standard potential (Ag/AgCl)) and the cathodic polarization curve of the current in the gold electroplating solution (12) of Comparative Example 4 were measured in the same way as in the gold electroplating solution (01) of Example 1 described above. These are Example 9 and Comparative Example 11, and are shown as dashed lines in
Next, test substrate 3 was immersed in gold electroplating solution (12) and Via-filling plating was performed at cathode current densities of 0.2 A/dm2 and 0.4 A/dm2. This gold electroplating solution (12) contained 30 mg/L of thallium formate (as thallium element) and 10 mg/L of bismuth nitrate (as bismuth element), even though the liquid composition deviated from that of the gold electroplating solution of the present invention. Subsequently, a glow discharge mass spectrometer (model: Astrum; manufactured by AMETEK) was used to analyze trace amounts of bismuth and thallium elements in the Via-filled gold deposits.
The thallium element in the resulting deposit was 2.76 ppm at a cathode current density of 0.2 A/dm2 and 4.61 ppm at a cathode current density of 0.4 A/dm2. On the other hand, none of the bismuth elements were detected. The content of the bismuth element was not detected, which is to say the amount of bismuth element was beneath the detection limit of 0.01 ppm. When the same gold electroplating was performed with a 5° C. lower liquid temperature, the thallium element content increased, but none of the bismuth element was detected.
The results show that in the comparative gold electroplating solution (12), the thallium element in the gold deposit increases as the cathode current density increases. On the other hand, the bismuth element was not found in the gold electrodeposits. The content of thallium elements is also influenced by the current density and liquid temperature of the gold electroplating solution.
Test substrate 1 was gold electroplated using gold electroplating solution (11). The gold electroplating solution (11) had 60 mg/L of thallium component, which is greater than the upper limit of 50 mg/L defined by the present invention, and also it (11) had 180 mg/L of bismuth component, which is also greater than the upper limit of 150 mg/L defined by the same. The electroplating operation was performed for 4 minutes. This is Comparative Example 13. In this, cross-sectional photographs of the inside of the drilled hole were taken at the end of the 4 minutes and are shown in
The gold-plated films of Va (diameter: 10 μm, depth: 10 μm), Vb (diameter: 5 μm, depth: 10 μm) and Vc (diameter: 3 μm, depth: 9 μm) in
The Au plating film in Vc of
Test substrate 1 was gold electroplated for 8 minutes using the same gold electroplating solution (11) as in Comparative Example 13. This is Comparative Example 14. A cross-sectional photograph of the interior of the drilled hole in Comparative Example 14 after 8 minutes is shown in
The gold plating films of Va, Vb and Vc of
Next, test substrate 1 and test substrate 2 were gold electroplated for 4 and 8 minutes using gold electroplating solutions (01), (02), and (04) through (16). The cross sections inside the drilled holes of test substrate 1 and test substrate 2 after 4 and 8 minutes were observed for each gold electroplating solution. These cross-sectional photographs (not shown) were similar to the cross-sectional photographs of the drilled holes (Va, Vb and Vc) in
Next, a gold film conductor circuit pattern was fabricated on the entire surface of a 3.0 mm thick silicon wafer substrate (200 mm diameter) to make Test Substrate 3 for comparative examples. The test substrate 3 was electroplated with seven different gold electroplating solutions (09) to (15) from Comparative Examples 15 to 21 by applying a DC current to the test substrate 3 for 8 minutes at a liquid temperature of 55° C. The thickness of the gold plating film on test substrate 3 was then measured in the same manner as in Examples, and the mean and standard deviation of the five locations were determined. The results of the average thickness and deviation of the seven gold-plated films of these Comparative Examples 15-21 are shown in Table 5.
As is seen from Table 5, there was only small difference in the average thickness and in its standard deviation in the case of the gold-plated film of test substrate 3. In particular, the average film thickness in the cases of the seven gold electroplating solutions (09) to (15) in Comparative Examples 15 to 21 ranged from 2.13 to 2.18 μm, with a standard deviation (3σ) ranging from 0.11 to 0.14.
Next, a palladium film conductor circuit pattern was fabricated on the entire surface of a 3.0 mm thick silicon wafer substrate (200 mm diameter) to make Test Substrate 4 for further comparative examples. The test substrate 4 was electroplated in the same manner as test substrate 3 for 8 minutes at a liquid temperature of 55° C. by applying DC current to seven different gold electroplating solutions (09) to (15) from Comparative Examples 22 to 28. The same was then measured as for the gold-plated film on test substrate 3, and the mean and standard deviation of the thickness of the palladium-plated film at the five locations were determined. These results are shown in Table 6.
As Table 6 shows, there was only small difference in the average thickness and in its standard deviation in the case of the gold-plated film of test substrate 4. In particular, the average film thickness in the cases of the seven gold electroplating solutions (09) to (15) in Comparative Examples 22 to 28 ranged from 2.13 to 2.17 μm, with a standard deviation (3σ) ranging from 0.10 to 0.12.
Next, comparison and study with regard to each of the above-mentioned properties of the gold electroplating solution among the Examples and Comparative Examples will be made.
It can be seen that the gold electroplating solutions (01) to (08) in Examples 1 to 8 and (09) to (18) in Comparative Examples 1 to 10 were all stable before and after the electroplating process, and no gold precipitates were observed after the electroplating process was completed. In short, there was scarce difference in the stability of the gold electroplating solution between Examples and Comparative Ones.
Now, comparison shall be made between the potential (−0.54 V) and current (−1.28 mA) at the inflection point of the solid line in
Comparison between the analysis results of electrodeposits in gold electroplating solution (01) and those in the case of gold electroplating solution (12) would suggest the following. Even though the gold electroplating process in Comparative Example 12 is similar to that in Example 7, the deposit in the drilled holes becomes different if the composition ratio of gold electroplating solution (12) and that of gold electroplating solution (01) is different. The analysis results about the electrodeposits in gold electroplating solution (12) of Comparative Example 12 show that the bismuth-catalyzed electrodeposition reaction depicted in
Next, a comparison is made between the cross-sectional photographs of
In all of the cross-sectional photos of Va through Vc of
Cross-sectional photographs of the interiors of the drilled holes in Va, Vb, and Vc in
In the case of test substrate 3, which has a gold film as the substrate surface, a comparison between Table 2 and Table 5 clearly shows that the mean values and standard deviations (3σ) of film thicknesses in the six gold electroplating solutions (01) to (06) of Examples 41 to 46 and those in the seven gold electroplating solutions (09) to (15) of Comparative Examples 15 to 21 are all within such narrow ranges that virtually no differences were observed between the Examples and Comparative Examples.
In the case of test substrate 4, which has a palladium plated film as the substrate surface, a comparison between Table 3 and Table 6 clearly shows that the mean values and standard deviations (3σ) of film thicknesses in the six gold electroplating solutions (01) to (06) of Examples 47 to 52 and those in the seven gold electroplating solutions (09) to (15) of Examples 22 to 28 are all within such narrow ranges that virtually no differences were observed between the Examples and Comparative Examples.
As shown above, the gold electroplating solutions (01) through (06) of the Examples have similar liquid stability as the gold electroplating solutions (09) to (15) of the Comparative Examples, and also the average and standard deviation of the gold plating film thickness of the Examples are good and acceptable. Moreover, the gold electroplating solution of the present invention has better Via-filling characteristics for gold deposition into the drilled hole than those of the comparative examples, indicating that it is preferable to preferentially involve the bismuth element in the gold deposit.
It has been confirmed that, in the gold electroplating solution of the present invention, the thallium catalyst and bismuth catalyst have deposition inhibiting and uniform deposition promoting effects, respectively, on the sulfite ion group present in an appropriate ratio, without the use of complex polymer compounds or surfactants, thereby inhibiting growth near the drilled hole opening and preferentially depositing on the interior, especially on the drilled hole's bottom. In other words, according to the present invention, the gold electroplating solution and the gold electroplating method are not affected by the current density distribution across the plated material, and hence a stable electroplating work is performed. Also, as shown at (b) of
According to the present invention, the gold electroplating solution and gold electroplating method can continuously pack gold deposits densely inside drilled holes by autonomous catalyst suppression reaction even if the current density distribution inside the perforation hole fluctuates irregularly during the electroplating operation. The presently invented gold electroplating solution and gold electroplating method are also widely applicable to various applications of existing Via-filling depositions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-152878 | Sep 2022 | JP | national |
